Looping in Reaper breaks my reverb plugin and now I'm confused

[krabbencutter]

Hey folks,

I have recently released an open source reverb plugin based on my Ursa Major Stargate 323. The plugin is built with JUCE and is available as CLAP & VST3 for Windows, macOS (Intel & M1) and Linux https://github.com/ravettel/SG-323/releases

Today one lucky tester discovered that looping in Reaper instantly creates large amounts of feedback and the automute will kick in. Interestingly enough this even happens in an empty session. As far as I can tell this seems to happen only in Reaper (tested in 6.68 & 6.71 on Windows & Linux) and looping in Bitwig 4 & ProTools 2022 (via VST3 wrapper) works absolutely fine.

I'd love to fix this but I currently have zero clue what's so different in Reaper's playback looping that my code will break o.O

[Lynx_TWO]

Hmmm, that doesn’t sound like the usual 2-pass render fix… are you sure FeedBack mode isn’t enabled in the Advanced tab under project settings? I recently ran into issues with that and had to find a workaround… only other thing I can think of off the top of my head is it might be a routing issue, but if you have the FX on the track itself then that’s not it…

What happens if you freeze the track or render the stem with the reverb turned off, then reenable/add the reverb FX on the frozen or rendered stem track?

[nofish]

There's a setting in Preferences (> Audio) > Playback: Flush FX when looping.
Maybe try if that makes a difference?

[krabbencutter]

I think I've found the reason why my plugin was acting weird. If the loop length can't be divided evenly through the current buffersize, Reaper will create resized audio buffers at the turnaround point of the loop. Say your buffersize is 512 samples, then Reaper might create a 320 sample buffer before the end of the loop and a 192 sample buffer after jumping back to the beginning of the loop. My plugin was assuming that the buffersize would stay constant during playback, which then could lead to a buffer overflow.